A New Petri-Net-Based Synthesis Technique for Supervisory Control of Discrete Event Systems

摘要

A new Petri-net-based top-down synthesis technique forsupervisory control of Discrete Event Systems (DES) is proposed tosolve the forbidden state problem. The supervisors obtained arecompiled supervisors, whose control policy is represented asa net structure, as opposed to mapping supervisors, whosecontrol policy is computed as a feedback function of the marking ofthe system. The compiled supervisors obtained by using the techniqueproposed in this paper are both nonblocking andmaximally permissive. The supervisors to be synthesisedconsist of a controlled Automation Petri Net model of thesystem. Automation Petri Nets (APN) include the following extensionsto the ordinary Petri net framework: sensor readings asfiring conditions at transitions and actions assigned toplaces. Ladder logic diagram (LLD) code is used to implement thesupervisors on programmable logic controllers (PLC). It is importantto note that the supervisors obtained are correct by construction;therefore there is no need for verification. The supervisory controlsynthesis technique proposed in this paper is applicable to bothhigh-level discrete event control, where the role of thesupervisor is to coordinate control of_-in the discrete manufacturingsense_-machines, workcells, etc., and low-level discrete eventcontrol, where the role of the supervisor is to arrange low-levelinteraction between control devices, such as motors and actuators. Inthis paper, the applicability of the proposed technique to low-leveldiscrete event control is demonstrated by considering an experimentaldiscrete manufacturing system.